The UK is to replace its stockpile of nuclear warheads at an estimated cost of more than £3bn, according to documents seen by the Guardian.

Ministers have repeatedly denied there are any plans to replace the warheads as part of the upgrade of the Trident nuclear system, insisting no decision will be taken until the next parliament, probably sometime after 2010.

However, previously unpublished papers released under the Freedom of Information Act reveal one of the MoD's senior officials told a private gathering of arms manufacturers that the decision had already been taken.

"This afternoon we are going to outline our plan to maintain the UK's nuclear deterrent," David Gould, then the chief operating officer at the Defence Equipment and Support Organisation, told a future deterrent industry day event. "The intention is to replace the entire Vanguard class submarine system. Including the warhead and missile."

According to the government's 2006 white paper, it would cost at least £3bn to replace the warheads, and opponents say the move would commit the UK to a nuclear weapons system for the next four decades.

Last night, peace campaigners said the new warheads would change the weapons' capabilities and may allow more targeted strikes, potentially making their use more likely.

"This document destroys any credibility in the government's claim that it has not yet made a decision on new nuclear warheads," said Kate Hudson, chair of the Campaign for Nuclear Disarmament. "It is a disgrace that the MoD is secretly telling the defence industry one thing, whilst ministers are saying quite the opposite to parliament."

The plans came to light after the MoD was forced to release Gould's "speaking notes" following a request under freedom of information legislation. In the initial release, defence officials blanked out the final sentence, referring to the warheads, because "the notes were incomplete information and therefore potentially misleading". That decision was overturned on appeal and the pivotal sentence was reinstated.

Yesterday, the Liberal Democrat defence spokesman, Nick Harvey, called on ministers to come clean about the government's plans for the country's nuclear deterrent. "Des Browne [the defence secretary] needs to urgently explain how the extract from this speech could so clearly contradict stated government policy on a new warhead ... This government promised an open and transparent debate about replacing Trident, but this feels more like the cloak and dagger days of the cold war."

A spokesman for the MoD said the document was a "speaking note" rather than a transcript of Gould's speech, which was delivered in June last year, adding that it did not reflect government policy.

"[The] decisions on whether and how to refurbish or replace our existing nuclear warhead are likely to be necessary in the next parliament ... No decisions have yet been taken."

Opponents say replacing the warheads would commit the UK to a nuclear weapons system up to 2055, as opposed to the lifespan of the current system, which is expected to become obsolete around 2025. They also claim that pressing ahead with a new generation of warheads before the non-proliferation treaty review conference in 2010 would be seen as inflammatory and could breach international agreements.

Harvey said: "Moving forward on a replacement warhead just two years before key talks on nuclear non-proliferation would be a decision with huge consequences and it demands open debate. The thought that it may have been taken already behind closed doors is deeply concerning."

Last year the government was forced to rely on Conservative party support to get its plans to renew Trident through parliament. Under those proposals, the nuclear submarines would be replaced and missiles upgraded, but no decision was taken on the warheads, which opponents say are the "key element" of any nuclear system.

"Building newer, potentially more advanced warheads will breach our commitment to disarm under the nuclear non-proliferation treaty and will send out a destabilising and hypocritical message to other states both with and currently without such weapons," said Hudson.

"A decision to go ahead with new warheads will have a much greater impact than the plan for new submarines, which merely provide the launch platform for these terrible weapons."